Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, especially eyeglass lenses. Optical materials, especially eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellow index, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Thus, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, a representative method uses an episulfide compound as described in Patent Documents 1 through 3.
However, an episulfide compound found in these patent documents is likely to generate an oligomer or a polymer depending on the reaction conditions, and as a result, the reaction yield is reduced. Therefore, optimization of the reaction conditions for producing an episulfide compound has been desired.
Regarding the reaction conditions for episulfidation, Patent Document 4 proposes a technique of allowing a reaction to proceed in a mixture solvent of a polar organic solvent and a non-polar organic solvent in the presence of an acid and/or an acid anhydride.
Patent Document 5 proposes a method of using only a polar solvent as a reaction solvent, and Patent Document 6 proposes a method of generating an isothiuronium salt as an intermediate substance in the presence of an acid. These methods both result in a low yield and are not practically usable. In addition, these methods require a large amount of acid and also a base for hydrolysis, and therefore require many steps and thus are disadvantageous also in the aspect of cost.
Patent Document 7 proposes a method of forming an epoxy compound into a corresponding isothiuronium salt and then performing hydrolysis thereon by use of ammonia or an ammonium salt. However, this method also requires a large amount of acid for generating the isothiuronium salt and a large amount of ammonia or ammonium salt for performing hydrolysis thereon. This complicates the production process, which is disadvantageous in the aspect of cost. In addition, since there is no description on the yield, it is not clear whether the method is practically usable or not.
As described above, for producing an episulfide compound, methods of adding an acid have been proposed. However, the methods described in these documents have been desired to be improved for the reasons that the acids used in these document mostly accompany a bad odor and thus require a measure against the odor, that the methods require a measure against corrosion, and that the methods may occasionally require a large amount of base for neutralization.